The present invention relates to an ultrasound diagnostic apparatus. Specifically, the present invention relates to an ultrasound diagnostic apparatus which allows stable display of a high-quality ultrasound image when various operations, such as freeze and change of image quality, are performed in the ultrasound imaging apparatus, a method of producing an ultrasound image, and a recording medium.
An ultrasound diagnostic apparatus using an ultrasound image has hitherto been put into practical use in the field of medicine.
In general, this type of ultrasound diagnostic apparatus is composed of an ultrasound probe (hereinafter, referred to as a probe) which has a piezoelectric element array in which piezoelectric elements to perform transmission and reception of ultrasonic waves are arranged, and a diagnostic apparatus body.
In the ultrasound diagnostic apparatus, an ultrasonic wave is transmitted from the probe toward a subject, an ultrasonic echo from the subject is received by the probe, and the reception signal is electrically processed in the diagnostic apparatus body to produce an ultrasound image.
In the piezoelectric element array of the ultrasound probe, an ultrasonic echo by single transmission of an ultrasonic beam is received by a plurality of piezoelectric elements. Accordingly, even if the ultrasonic echo is reflected at the same reflection point, the time until the ultrasonic echo enters each piezoelectric element differs depending on the position of the piezoelectric element.
For this reason, in the ultrasound diagnostic apparatus, delay correction using a delay time according to the position or the like of each piezoelectric element is performed on the reception signal output from the ultrasound probe, phase focusing is made by the delay correction, and phasing addition is performed to produce a sound ray signal (sound ray data), thereby producing an appropriate ultrasound image with no distortion.
The delay correction is performed using a sound speed of an ultrasonic wave (hereinafter, also simply referred to as “sound speed”) in the subject. In conventional the ultrasound diagnostic apparatus, on the assumption that the sound speed is constant, the value of the ultrasonic wave sound speed set as the whole of the apparatus is fixed to a certain value (for example, 1530 m/sec).
However, since the sound speed differs due to a difference of a tissue, such as a fat layer or a muscle layer in a living body, the sound speed of the ultrasonic wave in the subject is not uniform. Further, a stout subject and a thin subject are different from each other in the thickness of the fat layer or the muscle layer.
For this reason, in the conventional ultrasound apparatus in which the sound speed of the ultrasonic wave is fixed, the difference between the actual sound speed in the subject and the set sound speed often occurs.
If the set sound speed is different from the actual sound speed, it is not possible to accurately perform the delay correction. As a result, there is a problem in that image quality of the ultrasound image is deteriorated, for example, the produced ultrasound image is distorted with respect to the actual subject.
In contrast, JP 2011-92686 A describes an ultrasound diagnostic apparatus in which regions of interest are set so as to divide the inside of the subject (ultrasound image) into a plurality of regions, and a sound speed is set for each of the regions of interest. Specifically, in this apparatus, the transmission and reception of an ultrasonic wave for forming a transmission focus corresponding to a region of interest is performed, delay correction or phasing addition is performed by setting a plurality of sound speeds to calculate the focus index (for example, sharpness or the like) of the region of interest, and the sound speed at which the highest focus index is obtained is set as the sound speed in the region of interest.
According to an ultrasound diagnostic apparatus described in JP 2011-92686 A, an accurate sound speed is set corresponding to an individual difference of the subject, each of the sites in the subject, or the like, and delay correction is performed, thereby producing a high-quality ultrasound image with no distortion or the like.
The sound speed of the subject changes depending on the state of a tissue, such as a muscle. In addition, change in the position of the probe causes change in the sound speed of the subject in a region with which the probe is in contact. Accordingly, in order to stably obtain an ultrasound image of high-quality, it is desirable to appropriately update (reset) the sound speed.
On the other hand, in the ultrasound diagnostic apparatus, when setting the sound speed, many operations are required. That is, a frequent update of the sound speed imposes a heavy burden on the ultrasound diagnostic apparatus. For this reason, in the ultrasound diagnostic apparatus, the update of the sound speed is performed at a reasonable regular timing, for example, once at a predetermined time interval (once for every predetermined number of frames).
Meanwhile, as is well known, in the ultrasound diagnostic apparatus, in order to favorably perform the observation of the ultrasound image, various instructions can be given to the ultrasound image displayed on a display.
For example, while the ultrasound image is basically a motion image, that is, a live image, in the ultrasound diagnostic apparatus, in order to perform the observation of an intended site in detail, a FREEZE button (freeze switch) which instructs the display of a still image, instead of a motion image, is provided.
The ultrasound diagnostic apparatus is also provided with region of interest (ROI) setting means or observation depth setting means so as to instruct an observation target range (observation region) of the ultrasound image.
In addition, the ultrasound diagnostic apparatus is provided with gain (amplification and attenuation of the reception signal) adjustment means so as to change image quality (luminance) of the ultrasound image. An apparatus which can change image quality for each observation depth is also known.
Moreover, the ultrasound image has various image modes. For example, the ultrasound image has image modes such as a fundamental image mode in which according to a center frequency of an ultrasonic wave to be received, the center frequencies of ultrasonic waves to be transmitted and received are identical, a tissue harmonic mode in which an ultrasonic echo of a harmonic of a transmitted ultrasonic wave is received to produce an ultrasound image, a compound harmonic mode in which an ultrasonic echo having the same center frequency as that of a transmitted ultrasonic wave and a harmonic of the transmitted ultrasonic wave are received to perform image synthesis, and the like, and the ultrasound diagnostic apparatus is configured such that a desired mode can be selected and instructed.
Such various instructions (operations) are performed for some purpose.
For example, the instruction of freeze, the instruction to set an ROI, and the instruction to change an observation depth are made so as to observe a specific region in the subject in more detail. The change of image quality or the change of the image mode is instructed so as to observe an observation site in detail with an ultrasound image of higher-quality or an ultrasound image more suitable for the observation.
However, as described above, since a frequent update of the sound speed imposes a heavy burden on the ultrasound diagnostic apparatus, the update of the sound speed is performed at a reasonable regular timing. For this reason, in the ultrasound diagnostic apparatus, there are many cases where the sound speed which is set when various instructions are performed is different from the actual sound speed of the subject.
As a result, in spite of an ultrasound image desired to be observed in detail, deterioration of image quality, such as distortion, may occur in the displayed ultrasound image.